yz 125 stutter

my YZ used to do that, this might sound too obvious, but try cleaning the air filter thoroughly with simple green or dish soap, dry thoroughly, oil the filter and put it back on. if it still stutters and isn't crisp throughout the throttle range, run it sitting on the stand without an air filter (as long as it isn't windy and dusty, it's a good idea to cover the airbox holes with clean cloth while doing this, and if it sounds great, you probably need a new air filter. also try cleaning your in-tank fuel filter. another thing that helped me is repacking my silencer and soaking my pipe in diesel overnight and then swishing the diesel around inside the pipe as hard as I could before dumping it out (you have to plug both ends). you wouldn't believe how nasty that diesel was, it was like black soup. my bike never fouls plugs tho, that is in the carb more than likely.

You might wanna take a look at this

http://justkdx.dirtrider.net/carbtuning.html